Optical and optoelectronic modules of various types used in telecommunications and sensor industries typically utilize optical fibers to input or output light, or to transmit light between their internal optical components. Accordingly, providing a low-cost and efficient optical connection between an optical component and optical fiber is one of the most important requirements in designing fiber-coupled optical modules.
One type of such optical connection that is presently utilized to connect an optical component to an optical fiber is commonly referred to as an optical fiber “pigtail” connection. A fiber pigtail is a length of optical fiber, generally having a portion of its buffer coating removed and often coated with a metallized coating. Such pigtail connectors or couplings can optically couple an optical fiber or lensed optical fiber with light-producing or light-detecting elements coupled to leads of an integrated circuit. The light-producing element, for example, can be a semiconductor or diode laser, a frequency doubling crystal or a waveguide; a light-detecting element could be a photodiode. These elements are typically coupled with and fixedly disposed and oriented to an end of the lensed optical fiber, which is located in such a manner as to reside within the housing often forming a hermetic seal therewith. The lensed or collimating fiber extends beyond the edge of the housing and into it carrying signals to and/or from the integrated circuit. Mounting the fiber pigtail within the housing is not a trivial procedure. For example, alignment of a laser diode mounted within the housing with an end of the fiber pigtail must be usually such that maximum coupling of light exists from a laser to the fiber. Furthermore, the coupling must be robust, maintaining optimum alignment and coupling over time and often through changes in temperature and humidity. This coupling should preferably be tolerant of being handled or even dropped. Optical-coupling schemes that require high efficiency may utilize a lensed fiber that is attached to a substrate in close proximity to the light source, or discrete bulk lenses between the laser and the fiber. In both instances, the optical fiber is attached some minimum distance ˜0.5 mm away from the optical fiber termination.
One low-cost method of forming a relatively secure fixed connection between an optoelectronic component such as a laser diode and a photodiode and an optical fiber within a same package, is to coat a length near an end of an optical fiber pigtail with metal, and then solder it into position to a metal pad within a housing. For example, U.S. Pat. No. 6,146,025 discloses a package wherein an end of an optical fiber is stripped down to the cladding having a portion of the outer jacket removed and wherein the stripped clad portion is metallized and then attached to a metal pad in front of a laser with a solder ball.
This fiber mounting method has been used to mount conventional single-mode optical fibers in optical modules wherein the polarization state of the coupled light does not have to be preserved. In applications where polarization control between the optical component and a distal end of a fiber pigtail is required, the fiber pigtail is conventionally formed using a polarization-maintaining (PM) optical fiber. Applications of PM fibers include fiber Bragg grating (FBG) coupled pump lasers for erbium-doped, semiconductor and Raman optical amplifiers, and fiber-coupled frequency doubled lasers, wherein a pump laser can be coupled to a frequency doubling element such as a LiNbO3 waveguide with a PM fiber.
PM optical fibers differ from conventional optical fibers in that they transmit light while maintaining the polarization of light launched therein as the light propagates within the optical fiber, provided that the light is coupled into the PM fiber with it polarization aligned to one of two principal polarization axes of the PM fiber. These two principal axes are conventionally referred to as slow and fast axes, and typically result from a stress-induced birefringence in the PM fiber core. Due to this birefringence, optical signals polarized along these two axes will propagate with different speed with little coupling therebetween, so that the polarization of light is preserved when light propagates in the fiber. One common way to induce a permanent intrinsic stress so as to form a PM fiber is to provide two stress-inducing regions such as stress rods extending longitudinally within the optical fiber on opposite sides of the optical fiber core, as illustrated in FIG. P1 showing a cross-section of a PM fiber, which is commonly referred to as a Panda fiber, having a core 2 and two stress-inducing regions 4a and 4b. The slow axis 8 is oriented so as to connect centers of the stress rods 4a,b, while the fast axis 9 of the PM fiber is orthogonal to the slow axis 8. The stress-inducing regions 4a, 4b may be made of a material, for example boron-doped silicon oxide, which has a different coefficient of thermal expansion than those of the core 2 and clad 3 which are made of glass, so that a uniaxial tensile stress is applied to the core 2 in a plane of the stress rods 4a,b in a direction perpendicular to the longitudinal direction of the PM fiber, thereby defining the orientation of the slow axis 8.
The quality of a PM fiber in maintaining the polarization of light is conventionally characterized by the polarization extinction ratio (PER), which is the ratio of optical power of the light component in a dominant polarization state, typically corresponding to a linear polarization, to the optical power of the light component in the polarization state orthogonal thereto. The PER is conventionally measured in dB. It is typically desirable that the PER of light does not considerably degrade, i.e. is reduced as little as possible, between the input and output of a PM fiber. In other words, it is typically desirable that the PER of light at the output of a PM fiber is as high as possible.
Since the polarization maintaining quality of PM fibers relies on stresses within the optical fiber core, the polarization maintaining quality can be degraded by externally induced stresses associated with fiber bonding and soldering. Therefore, to make the external stresses associated with fiber bonding and mounting easier to control, such fibers have been traditionally mounted by soldering the PM fiber first inside a metal sleeve, and then bonding, for example soldering this sleeve with the PM fiber inside to a housing, since the external stresses associated with fiber soldering can be more symmetrical within a symmetrical sleeve, and therefore they are less likely to affect the built-in birefringence of the PM fiber, and therefore less likely to degrade the PER. Such PM fiber pigtail assemblies are disclosed in numerous publications including many US patents, for example, in U.S. Pat. Nos. 6,332,721 and 6,337,874 issued to Inokuchi and U.S. Pat. No. 6,400,746 issued to Yang, all of which disclose laser diodes utilizing PM fiber pigtail assemblies wherein the PM fibers are mounted within a cylindrical sleeve, which is then affixed to the laser housings.
In a typical prior art packaging procedure of a PM fiber pigtail, a portion of the PM fiber is surrounded with a solder preform and the solder preform is surrounded with a sleeve. The solder is then melted and allowed to solidify to secure the optical fiber within the sleeve. Historically, it has been often considered preferable to position a polarization maintaining fiber at the center of the packaging wherein the stresses on the cladding from the packaging are equalized, so to have the least effect on the optical fiber's PER from the packaging induced stresses as compared to other packaging configurations. However, one drawback of this procedure is that a PER degradation may occur because it is difficult to precisely position a PM fiber at the center of the sleeve. With the sign and the degree of the eccentricity of the fiber positioning within the sleeve being difficult to control and predict, the fiber PER can be degraded by the solder-induced stresses within the sleeve.
U.S. Pat. No. 6,480,675 issued to Dai and U.S. Pat. No. 6,782,011 issued to Kusano teach that this difficulty can be overcome by utilizing PM fiber mounting assemblies wherein a PM fiber is soldered within and elliptical or oval inner cavity of a sleeve or a ferrule, which holds a laser-coupled end of a PM fiber pigtail fixedly attached in a laser housing. By aligning the fast and slow axes of a PM fiber within the sleeve with one of the short and long axes of the elliptical cavity, so that the sleeve is symmetrical with respect to both the slow and fast axes of the PM fiber, the externally induced stress is made to be aligned with the internal PM fiber stress, e.g. the tensile stress induced in the fiber core 2 by the PM fiber stress rods 4a,b, as illustrated in FIG. P2 reproducing FIG. 9 of the '011 patent. In this figure, the PM fiber 1 is soldered within an oval solder reservoir 11b of a sleeve 10, which is referred to in the '011 patent as a ferrule.
To facilitate efficient coupling of the laser light into the PM fiber, a spherical or a chisel lens is often formed at the PM fiber end facing the laser, with a chisel having two inclined faces forming a ridgeline across the laser-facing end of the PM fiber. The chisel lens is typically formed by grinding the fiber end at an angle against a running grinder tape. After the formation of one inclined face, the fiber end is turned 180 degrees around its longitudinal direction, and the fiber end is again pressed against the running grinder tape to form the second inclined face. The '011 patent teaches to form the inclined faces so that the grinding removes cracks or flaws at the end of the PM fiber that may occur near the stress rods 4a,b during cleaving of the PM fiber. More particularly, the '011 patent teaches to grind the PM fiber end so that the stress-applied regions 4a, 4b are at the inclined faces of the chisel lens and are not exposed across the ridgeline 7. By selecting this orientation of the ridgeline 7 of the chisel lens at the end of the PM fiber relative to the stress rods 4a,b, a better quality of the chisel lens at the fiber end can be attained.
Accordingly, the prior art solutions to attaching a PM fiber pigtail within a housing of an optical module involve utilizing sleeves of ferrules, preferably with elliptical inner cavities as solder reservoirs, to hold the PM therein, and then affixing the sleeve with the PM fiber in it by bonding or soldering to the housing with the fiber proximate to an optical component to couple thereto, e.g. a laser. These types of pigtail assemblies, although appearing to perform their intended function, are however significantly more complex in fabrication than the direct soldering of fiber to a mounting pad, require non-standard components such as elliptical ferrules and sleeves to ensure that the PER is not degraded, and significantly add to the manufacturing costs of the final optical module.
An object of the present invention is therefore to overcome the shortcomings of the prior art by providing a low cost PM fiber pigtail assembly that preserves a high polarization extinction ratio of coupled light while utilizing a sleeve-less fiber mount.